An overload protector, which determines abnormal overload when overload of a predetermined load or more is exerted on a slide during pressurization with the slide of a pressing machine, and operates to allow the slide escape upward, is conventionally used. Here, as “a predetermined load or more”, for example, the case in which the load exceeds 10% or more of a rated load is cited. As the overload protector, the one disclosed in, for example, Japanese Utility Model Application Publication No. 61-24392 is known, and FIG. 4 shows an overload protector disclosed in Japanese Utility Model Application Publication No. 61-24392. In FIG. 4, a connecting rod 7 is connected to a plunger 19, which is provided at a frame 2 to be vertically movable, with a pin 8, and a step portion 19b is formed between a small diameter portion 19a formed at a lower part of the plunger 19 and an upper part of the plunger 19.
The lower part of the plunger 19 and the small diameter portion 19a are fitted in an upper hole of a slide 4, and an oil chamber 50 is formed between the step portion 19b of the plunger 19 and a step portion of the upper hole of the slide 4. An adjust screw 41 is screwed in the small diameter portion 19a from a lower surface to an upper position, and a gear 42 is formed at a lower part of the adjust screw 41. The gear 42 is meshed with a pinion 44 of a slide adjusting motor 45 attached to the slide 4 via an intermediate gear 43. The oil chamber 50 communicates with a port 51c of a change-over valve 51 via a conduit line 56. A port 51a of the change-over valve 51 is connected to a discharge side of a pump 52 via a check valve 58, and a port 51b of the change-over valve 51 communicates with a tank 57 via a check valve 55 which opens at set pressure. Further, the port 51c communicates with the tank 57 via an overload valve 53 and a check valve 54.
When the press is not pressurized, the change-over valve 51 is normally in a position a, and discharge oil from the hydraulic pump 52 enters the oil chamber 50 via the check valve 58, thus exerting constant preload pressure on the oil chamber 50. Pressing force at the time of driving the slide is transmitted to the slide 4 from the connecting rod 7 and the plunger 19 via oil pressure of the oil chamber 50. When overload is exerted on the slide 4, the oil pressure in the oil chamber 50 becomes predetermined pressure or more, and oil inside the oil chamber 50 escapes into the tank 57 via the overload valve 53, and the slide 4 escapes upward. At the time of adjustment of the slide, the change-over valve 51 is switched into a position b, and the oil pressure inside the oil chamber 50 is made lower than the aforementioned preload pressure, whereby load at the time of driving the slide adjusting motor 45 is reduced.
However, in the conventional hydraulic type of overload protector as described in the above-described Japanese Utility Model Application Publication No. 61-24392, overload is detected by utilizing a pressure change in the oil chamber 50 at the time of pressurization of the slide, and when the pressure of the oil chamber 50 rises to set pressure or more, oil in the oil chamber 50 is allowed to escape by means of the overload valve 53. Accordingly, due to a delay time in response of the hydraulic circuit thereof, a delay occurs before the slide 4 actually escapes upward, and thus it sometimes happens that overload protection does not sufficiently function when the slide 4 is operated at a high speed. Further, since the above-described load protector is installed, the number of components becomes larger, and an installation space is needed, which causes the disadvantage of raising the cost by that amount.